"We have flown with a video camera but do not have chemical sensors yet - they are being developed by other organisations."

What the team hopes is that sensors and communications equipment can be developed to be as light and as power-efficient as possible.

Very small sensor technology is being developed, and some nanotechnology techniques could be deployed to drastically improve the technology.

"There are always smart and nano materials being developed," said Dr Lind. "Currently, they require too much electricity to be useful. But they have great opportunities for the future."

The craft is powered by lithium ion batteries, but there lies the greatest challenge for small vehicles.

"Batteries are being developed all the time, though. Every month there are better ones on the market," he said.

"A lot of development is by the cellphone industry. As they develop, they slowly come to market." Finding small, low-cost circuit boards is also a problem right now.

Swarms and darts

Eventually, the craft will be tiny, allowing them to work in swarms, thus making them even more inconspicuous, the team believes.

"Colleagues have built vehicles as small as four inches across. They are difficult to spot visually. From an audio viewpoint, they are very quiet," said Dr Lind.

Working in swarms, each craft would communicate with each other. A likely scenario would involve a "mother ship" stationed high above a city, he explained.

"It could maybe fly 20 smaller vehicles inside the city. Each small one sends information up to the ship, which can then make decisions about the job and redirect the vehicles to other areas," he said.

One possibility for the morphing craft as they get smaller is using them to plant monitoring devices, such as microphones, into specific locations.

"You could have it shoot a small dart and the vehicle flies away, or the vehicle could change its shape to look like something else. The wings could fall off, for example."

Autonomously controlled drones will be ready in two to three years, Dr Lind said.

"They could be deployed very rapidly," said Dr Lind. "We are comfortable that the vehicles are of consistent quality that they will perform in a variety of conditions."

But in 20 years' time, the vehicles will go from bird size to insect size, the researcher believes. They will, at that point, be able to morph considerably, changing colour and form.

"They will be like biological systems so that they mimic birds much more than they do now."